int Search_Bin ( SqList L, ElemType key ) {
   low = 1;  high = L.length;     // 置区间初值
   while (low <= high) {
      mid = (low + high) / 2;
      if (key == L.elem[mid] )
          return  mid;        // 找到待查关键字
      else  if ( key < L.elem[mid])
          high = mid - 1;     // 继续在前半区间进行查找
      else  low = mid + 1; // 继续在后半区间进行查找
   }
   return 0;                 // 顺序表中不存在待查关键字
} // Search_Bin